1. Field of the Invention
The present invention relates to a built-in power supply unit for electronic devices that a battery pack is fitted to and, more particularly, to a power supply unit suitable for electronic devices such as personal digital assistants that switch to a backup power source when a battery pack is removed.
2. Description of the Related Art
Personal digital assistants such as cell-phones generally have a power supply unit that a battery pack is fitted to. The power supply unit has an insulating battery pack holder with a hollow portion to accommodate a battery pack detachably. A plurality of connector contacts formed of conductive metal are disposed inside the battery pack holder. Part of each connector contact protrudes into the hollow portion. These connector contacts are disposed in the positions corresponding to electrode terminals of the battery pack. When the battery pack is fitted into the hollow portion, the connector contacts are deformed elastically and pressed against the corresponding electrode terminals of the battery pack. The electrode terminals of the battery pack are thus conductively connected to a driver circuit through the connector contacts and power is supplied to the driver circuit.
Some of these types of personal digital assistants have a built-in backup power source in order to be usable even if the battery pack is removed. The backup power source is charged from the fitted battery pack and it discharges according to a change of an electrical signal caused by removal of the battery pack from the power supply unit.
However, the personal digital assistants that switch to the backup power source according to a change of an electrical signal caused by removal of the battery pack as described above have a problem. The problem is that the transmitting and receiving function is lost when the battery pack is replaced because power is not supplied from either the battery pack or the backup power source for a moment.
To solve this problem, a switch unit that can detect that the battery pack is almost completely removed and that can switch to the backup power source before the battery pack is removed completely is being studied. However, it is difficult, because of a restricted space, to add such a switch unit to the power supply unit for the electronic devices that require miniaturization. In addition, it increases the cost.